The present invention relates to a semiconductor laser, and more particularly to a semiconductor laser which can be modulated at ultrahigh speed used in optical communication or the like.
In recent years, demands for large capacity optical communication is becoming increasingly high. For this reason, a modulation speed higher than several tens of gigabits per second is demanded for semiconductor lasers which serve as the light sources. As a method for enhancing the response speed of the semiconductor laser there has been proposed to employ a quantum well structure or a strained quantum well structure having a high differential gain as an active layer. In fact, it is reported by Uomi et al. in Applied Physics Letters, Vol. 45, p. 818 (1984) that they succeeded in obtaining a relaxation oscillation frequency of over 10 GHz in a semiconductor laser using the GaAs/AlGaAs quantum well structure. Furthermore, it is reported in Applied Physics Letters, Vol. 51, p. 78 (1987) that the differential gain can be improved by accumulating holes in advance in a well layer by the use of a selectively p-doped structure for a multi-quantum well which becomes an active layer.
The response speed of a semiconductor laser is also be affected by the nonlinear gain. In order to improve the response speed it is necessary to reduce the nonlinear gain. Since reduction of the relaxation time within a band leads not only to a reduction in the nonlinear gain but also to a decrease in the differential gain at the same time, it will not act advantageously for the purpose of increasing the response speed.